This invention relates to electric switches and more particularly to switches which are used to switch very large direct currents such as the currents required for electromagnetic projectile launching.
One well known type of electromagnetic projectile launcher includes a pair of generally parallel conductive rails, a sliding armature for conducting current between the rails and propelling a projectile along the rails, a source of direct current, and a switching system for commutating current from the current source to the projectile launching rails. Current sources which employ an inductive energy storage element and a homopolar generator have demonstrated the capability of providing sufficient current to achieve an acceptable projectile acceleration. However, such inductively driven launcher systems require the service of an opening switch to accomplish the required power compression. The functions performed by the opening switch include: providing, in the closed position, a low resistance path for current flowing during the charging of the inductive energy storage device; and commutating, within a short time interval of typically less than 1 millisecond, the current flowing into the conductive rail load. In repetitive firing launcher systems, these functions must be performed in rapid succession.
For practical launcher systems, the peak current that must be commutated is in the order of several hundred thousand to several million amperes, required commutation time is about 1 millisecond or less and the inductive energy store has an inductance of a few microhenries. If a low-voltage device such as a homopolar generator is used as a prime power supply for charging the inductive energy store, a charging time in the order of several tens to several hundreds of milliseconds is required. This produces a very high accumulated amp.sup.2 -second (I.sup.2 t) during the inductor charging phase, in the order of 10.sup.11 amp.sup.2 second. The required performance parameters, that is, peak current, inductor charging time, accumulated I.sup.2 t and commutation time, create conflicting demands on the switch design and are the critical factors to be considered.
To reduce the resistive loss in the switch to within an acceptable range for the above-mentioned current magnitude and its associated accumulated I.sup.2 t and to be able to perform a repetitive switching function, a mechanical switch with multiple sliding metallic contacts is the preferred switch design. This type of switch conducts current through metal-to-metal interfaces between stationary and movable electrical contacts and can be designed to have a very low switch resistance, in the order of micro ohms or less, by increasing both the cross sectional area of the conductor and the number of contact points. However, this results in a massive movable contact which would inhibit the fast contact acceleration required to produce the rapidly rising arc voltage needed for current commutation.
One solution to this problem is illustrated in a copending, commonly assigned application Ser. No. 590,666, filed Mar. 19, 1984, and entitled "Repetitive Switch for Inductively Driven Electromagnetic Launchers", which is hereby incorporated by reference. That application illustrates a rotary switch which builds the movable contacts into a rotor that is rotated at a constant speed, thus not only eliminating the need to accelerate the contacts within a short duration but also achieving the repetitive switch open and close functions. The conflicting demands placed on the switch during inductor charging and while performing current commutation were satisfied by dividing the switch function between two parallel connected rotary switches which rotated at different speeds. The slow switch, which had massive contacts, was designed to conduct current during the inductor charging period and stays in the closed position until a few milliseconds before firing. At that time, the slow switch enters an open phase and commutates current into a fast switch. The fast switch, which was lighter and included a rotor rotating at a much higher speed, performed the current commutation into the load. Such a two switch system obviously requires close coupling between the fast switch rotor and the slow switch rotor to ensure the correct relationship between the conducting phase and the non-conducting phase so that the switches can perform their respective duties. Coordination of actuation between the sliding contacts of the two switches are also required. It is therefore desirable to provide an improved rotary switch design which performs all of the required switching functions in a single switch having a rotor rotating at a high speed. This would eliminate the need to connect two switches in parallel and provide a simple, more reliable switching system.